Cholinergic and noradrenergic stimulation in the rabbit flocculus have synergistic facilitatory effects on optokinetic responses

A recent study (Exp. Brain Res., 85 (1991) 475-481) showed that injection of the cholinergic agonist carbachol into the cerebellar flocculi had a pronounced facilitatory effect on the gains of the optokinetic (OKR) and vestibulo-ocular (VOR) reflexes, suggesting a positive modulatory role of the cholinergic system in the flocculus. Because many behavioral and electrophysiological studies throughout the brain have revealed a functional relationship between acetylcholine (ACh) and noradrenaline (NA), the present study was undertaken to compare the effects of floccular injection of the ACh agonist carbachol, the β-adrenergic agonist isoproterenol and the conjoint injection of both of these substances on the basic gain of the VOR and the OKR. Carbachol and isoproterenol both significantly raised the gain of the OKR, by 0.14 and 0.11 respectively. Neither of the two substances significantly affected the gain of the VOR in light or darkness. Conjoint injection of the same amounts of carbachol and isoproterenol resulted in an increase in the gain of the OKR by 0.29 without significant changes in the gains of the VOR in the light or in darkness. These results suggest a synergistic and positive modulatory role of ACh and NA in the flocculus.